The present invention generally relates to a video information transmission service system using an optical fiber network constituted by optical fiber cables, and particularly relates to a video information transmission service system having a head end device for frequency-multiplexing a plurality of video sources selected from a number of video sources at subscribers' requests, and for transmitting the selected video sources to a number of optical fibers which together constitute an optical fiber cable. The system further includes terminal means for requesting information from the head end device and selecting information on the basis of indication from the head end device. The invention also relates to the head end device to use in the system.
Conventionally, a video information transmission service system is known which uses an optical fiber cable, as a network, constituted by a number of optical fibers which have a broad band characteristic wide enough to have a large signal capacity. Briefly, this kind of animation image information service system has a head end device as a video information transmitting system for frequency-multiplexing and transmitting a plurality of video sources selected from a number of video sources at subscribers' requests, an optical fiber cable comprising a number of optical fibers for transmitting the frequency-multiplexed multi-channel signals transmitted by the head end device, and terminal means for receiving a picture and audio signal sent out from the head end device and transferred through the optical fiber cable.
FIG. 3 shows an example of a conventional configuration of the above-mentioned head end device. In FIG. 3, M (=l.times.m) video sources 1(1) to 1(l.times.m) are constituted by a video reproducing apparatus such as a VTR (video tape recorder), a VD (video disk), or the like. Pulse FM-modulators 2(1) to 2(l.times.m) respectively correspond to the video sources 1(1) to 1(l.times.m), for converting video signals from the corresponding video sources into pulse signals and for FM-modulating those pulse signals.
Digital switching means 3 acts as a video selecting means, having M (=l.times.m) input terminals into which video signals pulse-FM-modulated by the pulse FM modulators 2(1) to 2(l.times.m) are respectively fed. The digital switching means 3 is arranged to select arbitrary ones of the M video signals inputted to the input terminals and to output the selected video signals through arbitrary ones of N (=n.times.l) output terminals thereof. The digital switching means therefore has M.times.N (=m.times.n.times.l.sup.2) cross points.
A controller 4 determines through which of the N output terminals the arbitrarily selected ones of the M video signals received by the M input terminals of the digital switching means 3 are to be outputted. To this end, the controller 4 is constituted by a computer connected to subscribers through leased data communication circuits.
Up converters 5(1) to 5(l.times.m) are arranged respectively to receive video signals from the N (=n.times.l) output terminals of the digital switching means 3 and up convert the received video signals. The up converters 5(1) to 5(l.times.m) are divided into l groups each constituted by n up converters. The n up converters of each group divide a transmission band of an optical fiber into n parts and up convert the video signals into n different frequencies. The n signals up-converted by the n up converters of each group are put into n input terminals of corresponding ones of l mixers 6(1) to 6(l) which FM-multiplex the signals. The FM-multiplexed signals in the mixers 6(1) to 6(l) are put respectively into corresponding electro-optical (E/0) converters 7(1) to 7(l) and are converted into optical signals therein. The thus converted optical signals respectively are made incident into corresponding l optical fibers 8(1) to 8(l) constituting an optical fiber cable 8 and are transmitted therethrough.
Each of the l optical fibers 8(1) to 8(l) of the optical fiber cable 8 is led at its terminal to a corresponding optoelectric (0/E) converter 10 provided just before a distributor 9 as shown in FIG. 4. The optical signal transmitted through the optical fiber is converted into an electric signal in the 0/E converter 10. The electric signal converted by the 0/E converter 10 is distributed to a number of coaxial cables 11 by the distributor 9, and is transmitted through the coaxial cables 11 to terminal devices 12 installed in corresponding subscribers' houses.
Each terminal device 12 is constituted by a tuner 12a for selecting a desired channel from the multi-channel electric signal transmitted through the coaxial cable 11, a demodulator 12b for demodulating the signal of the channel selected by the tuner 12a, and a CPU 12c for controlling the channel selection operation of the tuner 12a and for performing data exchange with the above-mentioned head end device through a leased data communication circuit 13. A video apparatus 14 such as a television receiver, a monitor television or the like is connected to the output of the demodulator 12b, and an operating keyboard 15 for entering various data, such as a picture request, is connected to the CPU 12c.
Different frequencies are assigned to respective terminal devices in advance. To this end, a frequency-fixed down converter may be used in place of the tuner 12a.
In the above-mentioned configuration, video signals from n video sources are frequency-multiplexed and multi-channeled. The frequency-multiplexed and multi-channeled signals are transmitted through each optical fiber of the optical fiber cable 8 to the corresponding distributor 9 connected to the optical fiber through the corresponding 0/E converter 10. Those n video sources have been selected on the basis of requests transmitted to the controller 4 through leased data communication circuits from a number of terminal devices 12 connected with the distributor 9 through the coaxial cables 11.
As has been described above, in a head end device in the conventional video information transmission service system, for fibers constituting an optical fiber cable n channels in each fiber, and M (=m.times.l) video sources, the digital switching means 3 requires a number of cross points as expressed by the following equation, depending on the number of its input terminals M and the number N of its output terminals. ##EQU1##
Generally, the cost of the digital switching means 3 is proportional to the number of cross points thereof, and therefore an increase in the number of cross points raises cost greatly in a head end device and hence to a system.